Download 31. Deserts and Wind (p. 517-533)

Physical Geology 101
31. Deserts and Wind
(p. 517-533)
Wind
One of the major means of erosion, transport, and deposition of sediment is by wind. It may be an
important factor in any location where there is material light enough to blow around.
Locations:
1. ____________________
2. ____________________
3. ____________________
4. ____________________
5. _____________________________
Transport of Sediment by Wind
Any process that involves wind is called an _________________ process. Wind is a turbulent fluid and
therefore has many of the same transportation characteristics as water flowing in channels.
How dense is air compared to water? ___________________________________________
How does this affect the size of the sediment that air can carry?
______________________________________________________
Wind can suspend clay and silt size particles; very strong winds may be able to suspend small grains of
sand. Similar to rivers, the wind-transported sediment load can occur as:
1) ____________________ and
2) ____________________.
What two processes move the bed load?
1) __________________ - grains hop and bounce along in small jumps, like in water
2) ___________________ - grains of sand are rolled across the ground by the wind
Saltating grains are rarely lifted higher than about 1 meter. As individual grains hit the ground again, they
dislodge new grains and cause them to saltate too.
How high can suspended sediment be carried? ________________________
How far can suspended sediment be carried? _________________________
Examples of where sediment transport by wind has been significant:
1) ___________________________________
2) ___________________________________
3) ___________________________________
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Physical Geology 101
Erosion by Wind
Wind can erode in two ways: __________________ and ___________________.
Abrasion is due to saltating grains of sand impacting against a surface and causing small fragments to
chip off, whether it be the surface of another sand grain, a rock face, or some other surface.
Abrasion modifies the existing landscape. Individual sand grains become pitted, meaning that they have
small pits or gouges in them due to collisions with other grains. Rock surfaces may become smoothed by
abrasion, exhibiting a polished appearance.
As a rock fragment or stone gets abraded by the wind, the side facing the wind becomes polished and
faceted like a gemstone. These rock fragments are called ______________________.
If the stone rolls over at some point, or the wind direction changes, a new side faces the wind and a new
facet develops. The end result is a stone with several smooth, flat edges at various angles to each other.
In some deserts, long, elongate ridges of rock form aligned with the prevailing wind direction. They
resemble an upturned ship hull and are called _____________________.
Deflation is the removal of fine sediment by wind. It is most common where there is little vegetation or
where the surface sediment is not bound together. This may cause depressions or hollows to develop,
called _______________________________ or ________________________.
They may be small: a few meters across and less than 1m deep, or they may be many kilometers across
and several 10s of meters deep.
Example of a very large deflation basin: ________________________________ (Sahara)
Removal of fine sediment by deflation may also leave behind a surface of boulders, cobbles and pebbles
called a __________________________, which protects underlying fine sediment from further removal
by wind.
Deposition by Wind
Sediment carried by wind can be carried great distances before being deposited. It can then form a
variety of landforms, the most important ones being ________________ and ___________________.
Loess is a thick and uniform deposit of wind transported silt, sometimes containing small amounts of clay
and fine sand. The mineral grains can consist of quartz, feldspar, mica, and calcite. Loess is derived from
deserts, river floodplains, and recently glaciated surfaces.
How much of the Earth is covered by loess? ____________
How much of the U.S.A. is covered by loess? ____________
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Physical Geology 101
Development of Dunes
A dune is a mound or ridge of sand deposited by wind. Dunes form when wind blows around an obstacle
and deposits sand around it, which accumulates and builds up into a mound. As the mound gets bigger, it
acts as a barrier to the wind, causing it to slow down and deposit even more sand.
In profile, a dune is asymmetrical. The side facing the wind has a shallow slope and is called the
_________________ slope. Sand grains constantly saltate up this slope and then cascade over the top of
the dune and down the steeper slope, which slopes in the direction of the prevailing wind and is called the
_________________ slope or the __________________.
What is the angle of repose of the steep slope? ________________
If this angle is exceeded, sand tumbles to the base of the slope to re-attain the angle of repose. In this
way, dune sand is constant moving forwards, and the entire dune migrates in the direction of the
prevailing wind.
The cascading sediment forms a sedimentary structure within each bed of sediment called
____________________. Each cross-bed represents the position of the slip face in the past, before the
dune migrated forwards. This is why ancient sand dunes that are preserved in sandstone can be used to
determine ancient prevailing wind directions (called _____________________). The cross-beds are
always inclined downwards in the direction the wind was blowing.
Dune Types
What 4 factors control the shape of a sand dune?
___________________________
___________________________
___________________________
___________________________
The five dune types are:
•
______________________ – these are crescent shaped dunes, like giant croissants, with the tips,
called ______________, pointing in the direction the wind is blowing. They form on flat, dry
surfaces where there is little vegetation, a limited sand supply, and almost constant wind, and are
usually <30 m high.
How rapidly do they move or migrate? ____________________________
•
______________________ – also called ____________________. They form long, parallel ridges
aligned with the prevailing wind direction in regions of moderate sand supply. They usually form where
wind blows from two different directions, converging to form a prevailing wind. They can be up to 100m
high and >100 km long.
Where are they found? _____________________ and _____________________
•
______________________ – these form ridges aligned perpendicular to the prevailing wind direction in
areas where sand supply is high and vegetation is sparse. They are up to 200m high. From the air, they
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Physical Geology 101
resemble ocean waves and so are referred to as sand seas. Where sand supply diminishes, transverse
dunes become isolated and develop scalloped crests that somewhat resemble barchan dunes. Such
transverse dunes are called ____________________ dunes.
•
______________________ – these are common in coastal areas where there is ample sand supply
and vegetation. They are crescent shaped, like barchan dunes, but their horns point in the
direction the wind blows from. They can be distinguished from barchan dunes because the shallow
windward slope is on the inside of the crescent, and the horns and edges may be stabilized by
vegetation.
Where does the sand come from that forms the dune?
______________________________________________________
•
______________________ – these form in deserts where wind directions are variable. From above,
they resemble a star, with several ridges of sand radiating away from a central point. They may be up
to 100m high.
Do star dunes move around?
YES or NO
Deserts
How much of the Earth is desert? _________________
Warm air at the equator, produced by the sun’s rays hitting Earth with the greatest intensity, rises and
cools, producing rain. This is why equatorial regions are warm and wet. The dry air left behind then
moves northwards or southwards, getting cooler and more dense, causing it to sink. As it sinks, it gets
warmer and is perfect for the creation of a desert environment.
So what is the latitude of most of the world’s deserts? ________________
Types of Deserts
We can classify 5 different types of deserts:
•
_____________________ – the largest deserts, between 20-30° latitude.
Examples: ________________, _________________ and ______________________
•
_____________________ – areas so far from the ocean that air loses all its moisture before reaching
them. Example: ____________________
•
_____________________ – mountains block the flow of warm, moist air causing it to rise and produce
rain on the windward side but dry desert conditions on the leeward side of the mountains.
Examples: ____________________, ____________________ and ______________________
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Physical Geology 101
•
_____________________ – where cold ocean currents cool the air and lowers rainfall along the
coasts. Examples: ___________________ and ____________________
•
_____________________ – the polar regions are considered to be deserts because cold descending
air causes the small amount of precipitation to be in the form of snow, which accumulates to form the
ice caps. Example: _____________________
Desert Landforms
There are several types of landforms specific to deserts:
•
____________________ - temporary lakes after rainstorms
•
_______________ - dry lakebed of a playa lake, also called a ________________
•
________________________ and bajadas
•
____________________ - steep-sided erosional remnant sticking above the desert plain
•
____________________ - flat-topped erosional remnant with steep sides. A resistant layer produces
the flat top.
•
____________________ - a more eroded mesa that has become taller than it is wide
FINAL QUESTION:
What do we call an eroded butte that is much taller than it is wide?
______________________________
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